We are interested in the isolation and characterization of conditionally lethal mutations in mammalian cells in culture. Two classes are being studied: 1) Temperature-sensitive mutants defective in functions essential for cell proliferation and 2) respiration-deficient mutants. We hope that mutants in the first class will help us understand the regulation of the "biochemical differentiation" during the cell cycle, in particular the events which lead to the initiation of DNA synthesis and DNA synthesis itself. We have recently isolated a large number of Chinese hamster cell mutants defective in respiration. In two of these the defect has already been traced to defection or missing enzymes (NADH-CoQ reductase, succinate dehydrogenase) and in a third the defect appears to be at the level of mitochondrial protein synthesis. Further work is proposed: a) to determine complementation groups by somatic cell hybridization and b) to extend the biochemical characterizaton of many of these mutants. We hope that these studies will make interesting contributions to the problem of cellular energy metabolism and to our undestanding of the biogenesis of mitochondria.